Neutron Capture Measurements on 157Gd and 89Y at DANCE
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Date
2010-03-04
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Abstract
Neutron capture reactions are of crucial importance for different applications in nuclear physics and nuclear engineering. Common challenges in measuring these reactions are the separation of the $(n,gamma)$ channel from other reaction channels, such as $(n,el)$ and $(n,n')$, that contribute large backgrounds and obscure the $gamma$-ray cascade following neutron capture. Several major facilities focus on neutron capture. Perhaps the best detector for neutron capture measurements is the DANCE (Detector for Advanced Neutron Capture Experiments) array, a $4pi$ array of 160 BaF$_2$ crystals. In the present research two nuclei were investigated with DANCE: $^{157}$Gd and $^{89}$Y.
The $^{157}$Gd experiment was performed in 2006. This isotope is known for an enormous $(n,gamma)$ cross section -- the largest one in nature for stable isotopes. $^{157}$Gd has several practical applications, including its use as a shutdown system in nuclear reactors, medical therapy, neutron shielding, etc. There are 7 stable isotopes of Gd; this permits the study of the systematics of phenomena such as the scissors mode resonance as a function of mass and deformation. DANCE also enables the improvement of the resonance spectroscopy, including the level density, and thus the strength functions, essential for calculating neutron reaction rates.
$^{89}$Y was measured with DANCE in 2008, and additional data for improved statistics were taken in the following year. The main motivation was to improve the neutron capture cross section. Yttrium is important for stewardship science and its isotopes are used as radchem detectors to infer the neutron flux. The $^{89}$Y$(n,gamma)$ cross section is poorly known, with low accuracy and a limited neutron energy range. The major difficulty is that the elastic cross section is so large compared to neutron capture.
This thesis is divided into 3 parts. The 1$^{st}$ part contains a detailed description of the hardware, from the proton linac to DANCE, which is used in both experiments. The data acquisition system collects and performs an on-line analysis of the data on the event-by-event basis. The 2$^{nd}$ part is dedicated to the $^{157}$Gd experiment: how the data were taken, the challenges of the off-line analysis, and comparison of the DANCE results with other existing data, and with statistical model calculations. The 3$^{rd}$ part is about the $^{89}$Y experiment. This covers many of the same experimental topics as those for $^{157}$Gd. In addition this section explains how the neutron scattering background was simulated with other nuclei, and how the absolute neutron flux was determined with the aid of a resonance in $^{197}$Au.
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photo strength functions, neutron resonance, gadolinium, radiative neutron capture, Los Alamos, DANCE, cross section, yttrium
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PhD
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Physics
